Centering/positioning apparatus for wafer and vacuum chuck

ABSTRACT

A method for centering a semiconductor wafer on a vacuum chuck having a centered shaft. A shaft centering tool which is contained in pre-existing openings in a housing surrounding the shaft is used to define the desired location of the shaft. A second device having the same circumference as the wafer, is employed on the chuck in order that teflon guides can be set into position around the chuck. Once the guides are set, wafers introduced into the guides will be centered on the chuck.

This is a divisional of application Ser. No. 730,374 filed May 3, 1985,now U.S. Pat. No. 4,659,094.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of devices used to hold and spin awafer during the application of photoresist. More specifically, thisinvention relates to centering wafers on a vacuum chuck and positioningthe chuck/motor subassembly on which the semiconductor wafer restsduring the application of photoresist.

2. Prior Art

In the manufacture of semiconductor products, it is often desired toapply photoresist to a semiconductor wafer. Typically this is done byplacing the wafer on a platform where vacuum holds the wafer in place.This is referred to as a vacuum chuck. The chuck is mounted on a shaftwhich turns at approximately 5-6000 rpm. A certain amount of photoresistis placed on the spinning wafer and centrifugal force causes thephotoresist to spread evenly over the surface of the wafer. Should thewafer not be centered on the chuck properly, or the shaft is notpositioned properly, the photoresist will be thicker on one area of thesurface of the wafer than in other areas. This results in lower yieldsfrom that wafer since subsequent process steps are based on estimatedthickness of the photoresist layer. Additionally, if the wafer is notcentered, or the shaft is not positioned properly, excess vibrationcould eject the wafer from the chuck, further reducing yield.

In the prior art, wafers are centered on the chuck in the followingmanner. An aluminum wafer, of the same dimensions as the process wafersand having a hole in the center, is placed on the vacuum chuck. There isalso a hole in the center of the chuck itself. Using eyesight, theseholes are aligned, resulting in the wafer resting in the middle of thechuck. At that time, telfon guides are placed on either side of thewafer, tightened in place, and the aluminum wafer is removed. Duringprocessing, the teflon guides place wafers on the chuck. Each wafer isplaced on the chuck in the same location as the initial aluminum wafer.In order to operate effectively, the shaft/motor subassembly itself hasto be positioned and leveled. When the shaft assembly is not positionedand centered, an uneven distribution of the photoresist on the surfaceof the wafer results. In addition, there is vibration in the chuck andthe wafer which could eject the wafer from the chuck and result in loweryield. The shaft/motor assembly must be positioned, centered andlevelled each time the machine is disassembled. In the prior art, thiswas done by aligning the shaft by sight, reassembling the machine,checking the location of the shaft, disassembling the machine, placingthe shaft in a new location, reassembling the machine, again checkingthe location of the shaft by sight, and continuing the process until theshaft appears to be positioned and centered. Depending upon theexperience of the user, the machine had to be assembled and disassembled3 to 6 times with shaft centering and positioning taking 40 minutes toone hour to complete.

Disadvantages of the prior art wafer centering process are several. Afirst problem arises when the hole in the center of the wafer and thehole in the center of the chuck are not of the same diameter. If thehole in the wafer is larger than the hole in the chuck, the user mustattempt to center the chuck hole within the hole in the wafer. When thehole in the wafer is smaller than the hole in the chuck, the user isessentially guessing where the center of the chuck hole is when aligningthe wafer. Second, when placing the teflon guides around the wafer, theuser must hold the wafer with one hand, to prevent it from slipping offcenter, and work the guides with the other hand. Third, when tighteningthe guides, the wafer must still be held in place with one hand whilethe other tightens the guides.

The disadvantage of the prior art method of positioning the shaft/motorassembly is the obvious time waste in disassembling and reassembling thehousing repeated times, and if the final position is not correct, theuneven distribution of photoresist.

The present invention seeks to solve these problems with a simple yeteffective solution.

SUMMARY OF THE PRESENT INVENTION

The present invention consists of an aluminum standard which is usedinstead of a wafer when setting the teflon guides for wafer centering onthe vacuum chuck. The standard rests on the chuck and employs anoverhanging edge to hold the standard in place while the teflon guidesare moved and set. There is also a provision for the use of a screwinserted through the standard and into the center hole of the chuck, tohold the standard in place during centering.

In order to center and position the shaft/motor assembly and the chuck,a device is used which is placed on a pre-existing plate of the chuckhousing and is held in place by protrusions that fit into openings inthe plate. The device dimensions are such that the center line of theshaft is automatically aligned. The device also utilizes a longitudinalshaft guide to assure that the shaft is true.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a vacuum chuck and teflon guides.

FIG. 2 illustrates a top view of the shaft/motor assembly centering andpositioning device of the present invention.

FIG. 3 illustrates a side view of the shaft/motor assembly centering andpositioning device of the present invention.

FIG. 4 illustrates a perspective view of the wafer centering device ofthe present invention.

FIG. 5 illustrates a front view of the wafer centering device inoperation.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

A method for providing a centered wafer on a centered shaft/motorassembly during application of photoresist is described. A shaft/motorassembly centering tool is used to speed up and make more reliable thealignment of the shaft/motor assembly while a wafer centering toolprovides accurate centering of the wafer. In the following description,numerous specific details are set forth, such as wafer size, etc., inorder to provide a thorough understanding of the present invention. Itwill be obvious however, to one skilled in the art, that the inventionmay be practiced without these specific details. In other instances,well known structures have not been described in detail in order not tounnecessarily obscure the present invention.

When utilizing a vacuum chuck for holding wafers during the applicationof photoresist, it is necessary to center the wafers on the chuck foroptimum performance. Although the invention contemplates the use of onesize of wafer (e.g., 4 inch wafers), the wafer centering procedure mustbe performed repeatedly since 4 inch wafers come in differenttolerances. For example, due to the method of manufacture, the siliconingot from which silicon wafers are cut has a varying diameter.Typically there are several zones of a silicon ingot, each having adiameter falling in a certain range, but each used to make 4 inchwafers. The wafers may vary in size from 3.92 to 3.98 inches indiameter.

During photoresist processing of the wafers, a batch of wafers which allcame from the same area of the ingot are used at one time. When wafersfrom a different area are reached the teflon guides which hold the waferin place must be readjusted. If not, the wafers will not be centeredproperly, the photoresist will not be applied evenly, and the wafer mayvibrate and be ejected from the rotating vaccum chuck. For this reason,the wafers must be centered each time a new set of wafers from adifferent zone of the silicon ingot are used. To best deal with severaldifferent diameters of wafers, the present invention contemplates 2different sizes of wafer centering tools. One tool is used for wafers inthe range 3.92-3.95 and the other for wafers in the range 3.95-3.98. Theremainder of the discussion will refer to the tool used for the smallerwafers.

Referring to FIG. 1, the vacuum chuck 14 is shown as a circular platewith a shaft 15 attached. The chuck contains holes in the surface which,through the use of suction, will hold a wafer lying on the surface ofthe chuck. To center a wafer on the chuck, a test wafer is placed on thechuck in the center, and the teflon guides 11 and 12 are moved aroundthe chuck to contact the wafer and thus hold it in place. At that pointthe socket cap screws 13 are tightened, to set the location of theteflon guides 11 and 12. Once the guides are set, they move back andforth in the direction shown, each time setting a wafer on the chuck inthe same position as the previous wafer. Part of the housing of theapparatus includes a plate 16 which includes large holes 17 near theshaft and smaller holes 18 away from the shaft.

The wafer centering tool 20 is illustrated in FIG. 4. In the preferredembodiment, the device is aluminum and approximately 2.00 inches inlength. The outer edge of the tool 24 matches the outer circumference ofthat size of wafer which is to be used on the chuck. For the smallersize wafer, this dimension must be accurate within 2 thousandths of aninch for effective operation of the device. When the device contacts thechuck, the collar 21 surrounds the circumference of the chuck andprevents lateral movement of the tool. The mesa 22 contains a hole 23 inthe center. For most applications, the mass of the device is sufficientto hold it in place while the teflon guides are adjusted. However, ifneed be, a screw can be inserted through this hole 23 and into a hole inthe center of the chuck 14, the hole in the chuck being threaded. Inthis manner, the tool can be held firmly in place during centering andsetting of the teflon guides.

As shown in FIG. 5, the centering tool is placed on the chuck and isheld in place by the collar 21. The diameter of the circle formed by thecollar 21 matches the diameter of the chuck 14. This diameter on thetool must be accurate to within plus or minus 5 thousandths of an inchto prevent too much play in the device which could result in inaccuratecentering. The mesa area 22 must be flush with the chuck so the devicewhen centered will be level.

The time savings as a result of using the present invention aresubstantial. The prior art methods of centering the wafer, depending onthe skill of the operator, typically took 15 to 20 minutes. By utilizingthe present invention, the wafer guides can be placed in the properlocation in about 1 minute.

Shaft/Motor Assembly Centering Tool

The vacuum chuck is attached to the shaft 15 of FIG. 1 and the shaft isattached to motor 39. This shaft/motor assembly resides in a housing.This housing is disassembled for servicing on a regular basis as well aswhen wafer yields fall below a certain limit. Each time the housing isdisassembled, the shaft/motor assembly must be repositioned. In theprior art, this was done by eye, with the operator centering the shaftby sight, reassembling the housing, checking to see if the shaftassembly was positioned properly, disassembling the machine andadjusting the shaft, reassembling the machine to check the position ofthe shaft, and repeating this until the operator was satisfied that theshaft was positioned properly. An experienced operator typically had todisassemble and reassemble the machine several times which took between40 minutes and one hour.

Part of the housing remains stationary during all disassemblies andreassemblies. That part is the plate 16 shown in FIG. 1. The shaft/motorcentering tool of the present invention utilizes this plate to enable anoperator to center and position the shaft/motor assembly accurately thefirst time in 1 to 5 minutes.

The shaft/motor positioning tool is shown in FIGS. 1-3. The tool 30 iscomprised of pins 31, feet 32, shaft hole 33 and shaft brace 35. Thetool 30 takes advantage of the fact that the plate 16 is in a fixedlocation in the housing. When the housing is disassembled the tool 30 isplaced on the plate 16. The pins 31 fit into holes 18 of the plate whilethe feet 32 rest in holes 17 of the plate. In this manner the tool is ina fixed and repeatable position. Next the vacuum chuck 14 is removedfrom the shaft 15. Then the tool is placed on the shaft 15 and seated inthe holes 17 and 18. The tolerances of the tool are such that when theshaft 15 is inserted through the shaft hole 33 and shaft brace 35, theshaft is positioned correctly for operation. Although the shaft hole 33alone would be sufficient to center the shaft, the addition of the shaftbrace 35 insures that the shaft will be "true", that is, straight up anddown when in place. After centering, the shaft/motor assembly istightened in place and removed by sliding the tool 30 off the shaft andreplacing the chuck 14.

In the preferred embodiment, the shaft centering tool 30 is aluminum,although other materials such as stainless steel may be utilized. Theconstruction of the tool 30 is in 3 parts, a lower part containing thefeet 32 and pins 31 an upper part 34 and the shaft brace 35. The lowerand upper parts are held together by a screw inserted through the hole36 of the lower piece and entering the top piece 34. The shaft brace isattached by a screw through hole 37 to the upper part 34. It will beobvious, however, to one skilled in the art that other means ofattachment may utilized.

Thus, means have been described for positioning the shaft of a rotatingchuck used for the application of photoresist, and of centering asemiconductor wafer on the chuck of the shaft.

I claim:
 1. A device for defining a desired location for a shaft/motorassembly located in a housing having a plurality of fixed openingscomprising:a rigid member having a first and second end; a first crossmember orthogonally disposed on said first end; said first cross memberhaving a pair of first projections thereon, said first projectionsdemountably engaging a first pair of fixed openings in said housing; asecond cross member orthogonally disposed on said second end; saidsecond cross member having a pair of second projections thereon; saidsecond projections demountably engaging a second pair of fixed openingsin said housing; said second cross member defining a first aperturetherethrough for retaining said shaft of said shaft/motor assembly, saidfirst aperture coincident with said desired location when saidprojections engage said fixed openings; whereby said shaft/motorassembly may be easily placed at said desired location.
 2. The device asdefined by claim 1 further including:a raised member attached to saidrigid member; an arm attached to said raised member and extending oversaid first aperture, said arm defining a second aperture coincident withsaid first aperture for providing longitudinal positioning of saidshaft.
 3. The device as defined by claim 2 wherein said raised member isattached to said rigid member by a bolt extending through said base andthreadedly engaging a tapped hole in said raised member.
 4. The deviceas defined by claim 3 wherein said arm is attached to said raised memberby means of a bolt extending through said arm and threadedly engagingsaid tapped hole.